Traffic barrier
Encyclopedia
Traffic barriers keep vehicle
s within their roadway and prevent vehicles from colliding with dangerous obstacle
s. Traffic barriers installed at the road side also prevent errant vehicles from traversing steep (non-recoverable) slopes. Traffic barriers installed at the medians of divided highways are also referred to as median barriers. The latter also prevent errant vehicles from entering the opposing carriageway
of traffic and help to prevent head-on collisions.
Clearzone also known as clear recovery area or horizontal clearance is defined (through study) as a lateral distance in which a motorist on a recoverable slope may travel outside of the travelway and return their vehicle safely to the roadway. This distance is commonly determined as the 85th percentile in a study comparable to the method of determining speed limits on roadways through speed studies and varies based on the classification of a roadway. In order to provide for adequate safety in roadside conditions, hazardous elements, whether they be obstacles or steep slopes can be placed outside of the clearzone in order to reduce or eliminate the need for roadside protection.
When barrier is needed, careful calculations are completed to determine length of need which take into account the aforementioned factors. Specifically, the traffic volumes and therefore, the classification of the roadway in addition to the distance of the hazard from the edge of travelway and the distance or offset of the barrier to be placed or installed from the edge of travelway. It is the case in current times, that barrier or rail that is to be used in construction and maintenance operations has undergone extensive testing in both government and private research facilities in order to determine proper 'crash-worthiness' and effectiveness in conditions which are prescribed for its use. In particular, most roadside protection, whether it be a concrete barrier or rail, or a metal beam fence will perform properly only when placed in adequate proximity to the travelway so as to prevent vehicle impacts at large (obtuse) angles. The method in which a barrier protects motorists from roadside hazards is in how it dissipates the energy of an impact.
crashes, fixed objects like bridge piers, and bodies of water. Roadside barriers can also be used wide medians, to prevent vehicles from colliding with hazards within the median.
Median barriers are used to prevent vehicles from crossing over a median and striking an oncoming vehicle in a head-on crash. Unlike roadside barriers, they must be designed to be struck from either side.
Bridge barrier is designed to restrain vehicles from crashing off the side of a bridge and falling onto the roadway, river or railroad below. It is usually higher than roadside barrier, to prevent trucks, buses, pedestrians and cyclists from vaulting or rolling over the barrier and falling over the side of the structure. Bridge rails are usually multi-rail tubular steel barriers or reinforced concrete parapets and barriers
Work zone barriers are used to protect traffic form hazards in work zones. Their distinguishing feature is they can be relocated as conditions change in the road works. Two common types are used: temporary concrete barrier and water-filled barrier. The latter is composed of steel-reinforced plasic boxes that are put in place where needed, linked together to form a longitudinal barrier, then ballasted with water. These have an advantage in that they can be assembled without heavy lifting equipment, but they cannot be used in freezing weather.
, traffic barriers are tested and classified according to the AASHTO Manual for Assessing Safety Hardware (MASH) standards, which recently superseded Federal Highway Administration
NCHRP Report 350. Barrier deflections listed below are results from crash test
s with a 2000 kg pickup truck traveling 100 km/h, colliding with the rail at a 25 degree angle.
Flexible barriers include cable barriers and weak post corrugated guide rail systems. These are referred to as flexible barriers because they will deflect 1.6 m to 2.6 m when struck by a typical passenger car or light truck. Impact energy is dissipated through tension in the rail elements, deformation of the rail elements, posts, soil and vehicle bodywork, and friction between the rail and vehicle.
Semi-rigid barriers include box beam guide rail, heavy post blocked out corrugated guide rail and thrie-beam guide rail. They deflect three to six feet: more than rigid barriers, but less than flexible barriers. Impact energy is dissipated through deformation of the rail elements, posts, soil and vehicle bodywork, and friction between the rail and vehicle. Box beam systems also spread the impact force over a number of posts due to the stiffness of the steel tube. These barrier systems deflect up to 1.5 m.
Rigid barriers are usually constructed of reinforced concrete. A permanent concrete barrier will only deflect a negligible amount when struck by a vehicle. Instead, the shape of a concrete barrier is designed to redirect a vehicle into a path parallel to the barrier. This means they can be used to protect traffic from hazards very close behind the barrier, and generally require very little maintenance. Impact energy is dissipated through redirection and deformation of the vehicle itself. Jersey barriers and F-shape barriers also lift the vehicle as the tires ride up on the angled lower section. For low-speed or low-angle impacts on these barriers, that may be sufficient to redirect the vehicle without damaging the bodywork. The disadvantage is there is a higher likelihood of rollover with a small car than the single slope or step barriers. Impact forces are resisted by a combination of the rigidity and mass of the barrier. Deflection is usually negligible.
An early concrete barrier design was developed by the New Jersey
State Highway Department. This led to the term Jersey barrier
being used as a generic term, although technically it applies to a specific shape of concrete barrier. Other types include constant slope barrier
s, concrete step barrier
s and F-shape barrier
s.
Concrete barriers usually have smooth finishes. At some impact angles, coarse finishes allow the drive wheel of front wheel drive vehicles to climb the barrier, potentially causing the vehicle to roll over. However, along parkway
s and other areas where aesthetics are considered important, reinforced concrete walls with stone veneers or faux stone finishes are sometimes used. These barrier walls usually have vertical faces to prevent vehicles from climbing the barrier.
Vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....
s within their roadway and prevent vehicles from colliding with dangerous obstacle
Obstacle
An obstacle is an object, thing, action or situation that causes an obstruction. There are, therefore, different types of obstacles, which can be physical, economic, biopsychosocial, cultural, political, technological or even military.-Physical barriers:As physical obstacles, we can enumerate all...
s. Traffic barriers installed at the road side also prevent errant vehicles from traversing steep (non-recoverable) slopes. Traffic barriers installed at the medians of divided highways are also referred to as median barriers. The latter also prevent errant vehicles from entering the opposing carriageway
Carriageway
A carriageway consists of a width of road on which a vehicle is not restricted by any physical barriers or separation to move laterally...
of traffic and help to prevent head-on collisions.
Need and placement
Traffic barriers constitute hazards themselves and should only be used when the obstacle poses a greater threat than the barrier itself. In all cases, roadside hazards must be assessed for the danger they pose to traveling motorists based on size, rigidity and distance from the edge of travelway. For instance, small roadside signs and some large signs (ground-mounted breakaway post) often do not merit roadside protection as the barrier itself may pose a greater threat to general health and well-being of the public than the obstacle it intends to protect. In many regions of the world, the concept of clearzone is taken into account when examining the distance of an obstacle or hazard from the edge of travelway.Clearzone also known as clear recovery area or horizontal clearance is defined (through study) as a lateral distance in which a motorist on a recoverable slope may travel outside of the travelway and return their vehicle safely to the roadway. This distance is commonly determined as the 85th percentile in a study comparable to the method of determining speed limits on roadways through speed studies and varies based on the classification of a roadway. In order to provide for adequate safety in roadside conditions, hazardous elements, whether they be obstacles or steep slopes can be placed outside of the clearzone in order to reduce or eliminate the need for roadside protection.
- Common sites for installation of traffic barrier:
- Bridge ends
- Near steep slopes from roadway limits
- At drainage crossings or culvertCulvertA culvert is a device used to channel water. It may be used to allow water to pass underneath a road, railway, or embankment. Culverts can be made of many different materials; steel, polyvinyl chloride and concrete are the most common...
s where steep or vertical drops are present - Near large signs/illumination poles or other roadside elements which may pose hazards
When barrier is needed, careful calculations are completed to determine length of need which take into account the aforementioned factors. Specifically, the traffic volumes and therefore, the classification of the roadway in addition to the distance of the hazard from the edge of travelway and the distance or offset of the barrier to be placed or installed from the edge of travelway. It is the case in current times, that barrier or rail that is to be used in construction and maintenance operations has undergone extensive testing in both government and private research facilities in order to determine proper 'crash-worthiness' and effectiveness in conditions which are prescribed for its use. In particular, most roadside protection, whether it be a concrete barrier or rail, or a metal beam fence will perform properly only when placed in adequate proximity to the travelway so as to prevent vehicle impacts at large (obtuse) angles. The method in which a barrier protects motorists from roadside hazards is in how it dissipates the energy of an impact.
Barrier types and performance
Traffic barriers are categorized in two ways: by the function they serve, and by how much they deflect when a vehicle crashes into them.Barrier functions
Roadside barriers are used to protect traffic from roadside obstacles or hazards, such as slopes steep enough to cause rolloverRollover
A rollover is a type of vehicle accident in which a vehicle tips over onto its side or roof. The most common cause of a rollover is traveling too fast while turning.- Dynamics :Vehicles can roll over in several ways...
crashes, fixed objects like bridge piers, and bodies of water. Roadside barriers can also be used wide medians, to prevent vehicles from colliding with hazards within the median.
Median barriers are used to prevent vehicles from crossing over a median and striking an oncoming vehicle in a head-on crash. Unlike roadside barriers, they must be designed to be struck from either side.
Bridge barrier is designed to restrain vehicles from crashing off the side of a bridge and falling onto the roadway, river or railroad below. It is usually higher than roadside barrier, to prevent trucks, buses, pedestrians and cyclists from vaulting or rolling over the barrier and falling over the side of the structure. Bridge rails are usually multi-rail tubular steel barriers or reinforced concrete parapets and barriers
Work zone barriers are used to protect traffic form hazards in work zones. Their distinguishing feature is they can be relocated as conditions change in the road works. Two common types are used: temporary concrete barrier and water-filled barrier. The latter is composed of steel-reinforced plasic boxes that are put in place where needed, linked together to form a longitudinal barrier, then ballasted with water. These have an advantage in that they can be assembled without heavy lifting equipment, but they cannot be used in freezing weather.
Barrier stiffness
Barriers are divided into three groups, based on the amount they deflect when struck by a vehicle and the mechanism the barrier uses to resist the impact forces. In the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, traffic barriers are tested and classified according to the AASHTO Manual for Assessing Safety Hardware (MASH) standards, which recently superseded Federal Highway Administration
Federal Highway Administration
The Federal Highway Administration is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two "programs," the Federal-aid Highway Program and the Federal Lands Highway Program...
NCHRP Report 350. Barrier deflections listed below are results from crash test
Crash test
A crash test is a form of destructive testing usually performed in order to ensure safe design standards in crashworthiness and crash compatibility for various modes of transportation or related systems and components.- Types :...
s with a 2000 kg pickup truck traveling 100 km/h, colliding with the rail at a 25 degree angle.
Flexible barriers include cable barriers and weak post corrugated guide rail systems. These are referred to as flexible barriers because they will deflect 1.6 m to 2.6 m when struck by a typical passenger car or light truck. Impact energy is dissipated through tension in the rail elements, deformation of the rail elements, posts, soil and vehicle bodywork, and friction between the rail and vehicle.
Semi-rigid barriers include box beam guide rail, heavy post blocked out corrugated guide rail and thrie-beam guide rail. They deflect three to six feet: more than rigid barriers, but less than flexible barriers. Impact energy is dissipated through deformation of the rail elements, posts, soil and vehicle bodywork, and friction between the rail and vehicle. Box beam systems also spread the impact force over a number of posts due to the stiffness of the steel tube. These barrier systems deflect up to 1.5 m.
Rigid barriers are usually constructed of reinforced concrete. A permanent concrete barrier will only deflect a negligible amount when struck by a vehicle. Instead, the shape of a concrete barrier is designed to redirect a vehicle into a path parallel to the barrier. This means they can be used to protect traffic from hazards very close behind the barrier, and generally require very little maintenance. Impact energy is dissipated through redirection and deformation of the vehicle itself. Jersey barriers and F-shape barriers also lift the vehicle as the tires ride up on the angled lower section. For low-speed or low-angle impacts on these barriers, that may be sufficient to redirect the vehicle without damaging the bodywork. The disadvantage is there is a higher likelihood of rollover with a small car than the single slope or step barriers. Impact forces are resisted by a combination of the rigidity and mass of the barrier. Deflection is usually negligible.
An early concrete barrier design was developed by the New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
State Highway Department. This led to the term Jersey barrier
Jersey barrier
A Jersey barrier or Jersey wall is a modular concrete barrier employed to separate lanes of traffic. It is designed to both minimize vehicle damage in cases of incidental contact while still preventing crossover in the case of head-on accidents....
being used as a generic term, although technically it applies to a specific shape of concrete barrier. Other types include constant slope barrier
Constant Slope Barrier
A constant slope barrier is a traffic barrier made of reinforced concrete and designed with a single slope that is used to separate lanes of vehicular traffic...
s, concrete step barrier
Concrete step barrier
A concrete step barrier is a safety barrier used on the central reservation of motorways and dual carriageways as an alternative to the standard steel crash barrier.The barrier has contained all vehicles up to 13.5 tonnes.-United Kingdom:...
s and F-shape barrier
F-Shape Barrier
The F-shape barrier is a concrete crash barrier, originally designed to divide lanes of traffic on a highway. It is a modification of the widely-used Jersey barrier design, and is generally considered safer....
s.
Concrete barriers usually have smooth finishes. At some impact angles, coarse finishes allow the drive wheel of front wheel drive vehicles to climb the barrier, potentially causing the vehicle to roll over. However, along parkway
Parkway
The term parkway has several distinct principal meanings and numerous synonyms around the world, for either a type of landscaped area or a type of road.Type of landscaped area:...
s and other areas where aesthetics are considered important, reinforced concrete walls with stone veneers or faux stone finishes are sometimes used. These barrier walls usually have vertical faces to prevent vehicles from climbing the barrier.